Hello Ian (is that right?) 
I hope you are feeling good after your surgery. Congratulations on starting your Bikram Yoga journey.
Correct me if I am wrong but it seems that your bottom is not on the floor and you are taking yourself back part of the way and using your arms to prop you up.
You know, I think you may be going too far with this pose at the moment. A rule that is never transgressed in my studio is this: if you can’t get your butt on the floor between your heels, then you must stay seated and upright.
Stay completely upright, vertical. Your hips must always touch your heels even if your hips are not on the floor. There should be no separation so you can avoid over twisting the knees. Your ankles, knees and hips obviously need some more time in this position. And the best way is to harness the forces of gravity to help you. The moment you incline your back backwards you remove much of the powerful vertical vector of force that is driving directly downward through your spine and hips and ankles helping to open everything up.
If you sit backward, then due to the tightness in those areas your hips are likely to lift a little and you are likely not to benefit at all. There are a myriad of other problems with going back early but I will leave those for now. But the one that is clearly bothering you is strain on the back. You may even have some strain in your neck as a result.
One thing you can try is to sit with your hips and heels in the right position and draw your knees further apart. This can lower your hips. Make sure that the hips always touch the heels and that there is a straight line from your knees to your toes regardless of leg angle (which will change if you move your knees apart).
The good news is that even the very inflexible yogis who start in this fashion may not take very long to open and awaken their bodies. It is quite common for these folk to be lying down on the floor looking comfortable - you’d never guess that they had trouble before!
It can be hard to stay put when everyone else is ‘progressing’ beyond the first stage and if it looks as though that is what you are supposed to do. Patience will yield you very much better results.
What you are working towards is having your butt sitting on the floor, not just skimming the floor. Then, and only then should you progress: by putting hands on feet, fingers forward and going back onto your elbows.
What you are doing to get into those other poses is perfect. Only you can feel the vulnerability in your back. Hopefully as you heal further you will build some good strength in your core to support you even better in your forward bends.
Keep me posted on your progress.
Namaste
Gabrielle 
